Your father's Marantz 2230 receiver can be the control center of a very nice system. The 2230 uses the same amplifier as the highly regarded Marantz model 1060 integrated amplifier. The 2230 can be used with any dynamic (non-electrostatic) headphone made today and will work especially well with the higher impedance headphones from manufacturers such as Sennheiser and AKG.
The 2230 had a suggested retail price of $350 back in 1972. In equivalent dollars, the 2230 would cost more than $1,500 to build and market today.
You would have to spend hundreds of dollars today to get a dedicated solid state headphone amplifier that would sound better than the Marantz. Also, the dedicated headphone amplifiers generally only have one or at most two high level inputs, no phono stage, no tone controls, and no AM/FM tuner as does the 2230.
The reason one channel keeps cutting out is probably that the contacts on one or more of the push button switches are dirty. This is usually easy and inexpensive to fix. You will need a can of compressed air, such as Dust-Off. You can get this at any office supply superstore, computer store, Best Buys, or Radio Shack. Radio Shack also sells a can of tuner cleaner spray for about $8. You will need this cleaner spray to complete the following steps.
First step, unplug the receiver from the wall outlet.
Then remove the screws holding the cover in place and remove the cover.
Carry the unit outside or into the garage for the next step. Once outside the house, use the can of compressed air to blow out all of the dust inside the chassis and the from around and inside the buttons, controls, and switches.
Once you have blown out any dust you see, you take the tuner cleaner spray, attach the extension nozzle to the can and and spray each control, switch, and button with one or two quick blasts from the spray can
Then work each control for a minute, working the spray into the contact surfaces.
Note: To get to some of the switches and controls, the bottom plate may also have to be removed.
Wait about 15 minutes before plugging the unit back into the power outlet. Test all controls for smooth and silent operation. Some controls may require a second treatment before they start to operate smoothly and with minimal static.
Unplug the receiver to reinstall the bottom plate and the case, and your 2230 receiver should be good to go for many more years. For best FM performance, you may want to have the FM tuner I.F. stages realigned as these can drift out of alignment over time. This can be done by most electronics repair shops for under $50.
The most problem prone part on these old Marantz units is the power switch. If your switch still works, good, just don't use it all the time to turn the unit on and off. Get a power strip/surge protector and use the switch on this strip to power the Marantz unit on and off.
Good luck with your Marantz 2230.